System and Method for Optimizing Reactive Power Generation of a Wind Farm

ABSTRACT

A method for optimizing reactive power generation of an electrical power system includes generating, via a plurality of cluster-level controllers, a cluster-level reactive power command for each cluster of electrical power subsystems based on a system-level reactive power command. The method also includes determining, via the cluster-level controllers, a subsystem-level reactive power command for each of the electrical power subsystems based on the cluster-level reactive power command. Further, the method includes evaluating, via a plurality of subsystem-level controllers, reactive power capability of a plurality of reactive power sources within each of the electrical power subsystems. Moreover, the method includes generating, via each of the subsystem-level controllers, an actual reactive power for each of the electrical power subsystems based on the evaluation by allocating a portion of the subsystem-level reactive power command to each of the plurality of reactive power sources.

FIELD

The present disclosure relates generally to systems and methods foroperating wind farms, and more particularly, to systems and methods foroptimizing reactive power (VAR or Q) generation of a wind farm arrangedin a plurality of wind turbine clusters.

BACKGROUND

Wind power is considered one of the cleanest, most environmentallyfriendly energy sources presently available, and wind turbines havegained increased attention in this regard. A modern wind turbinetypically includes a tower, a generator, a gearbox, a nacelle, and oneor more rotor blades. The rotor blades capture kinetic energy of windusing known airfoil principles. For example, rotor blades typically havethe cross-sectional profile of an airfoil such that, during operation,air flows over the blade producing a pressure difference between thesides. Consequently, a lift force, which is directed from a pressureside towards a suction side, acts on the blade. The lift force generatestorque on the main rotor shaft, which is geared to a generator forproducing electricity.

For example, FIGS. 1 and 2 illustrate a wind turbine 10 and associatedpower system suitable for use with the wind turbine 10 according toconventional construction. As shown, the wind turbine 10 includes anacelle 14 that typically houses a generator 28 (FIG. 2). The nacelle 14is mounted on a tower 12 extending from a support surface (not shown).The wind turbine 10 also includes a rotor 16 that includes a pluralityof rotor blades 20 attached to a rotating hub 18. As wind impacts therotor blades 20, the blades 20 transform wind energy into a mechanicalrotational torque that rotatably drives a low-speed shaft 22. Thelow-speed shaft 22 is configured to drive a gearbox 24 (where present)that subsequently steps up the low rotational speed of the low-speedshaft 22 to drive a high-speed shaft 26 at an increased rotationalspeed. The high-speed shaft 26 is generally rotatably coupled to agenerator 28 (such as a doubly-fed induction generator or DFIG) so as torotatably drive a generator rotor 30. As such, a rotating magnetic fieldmay be induced by the generator rotor 30 and a voltage may be inducedwithin a generator stator 32 that is magnetically coupled to thegenerator rotor 30. The associated electrical power can be transmittedfrom the generator stator 32 to a main three-winding transformer 34 thatis typically connected to a power grid via a grid breaker 36. Thus, themain transformer 34 steps up the voltage amplitude of the electricalpower such that the transformed electrical power may be furthertransmitted to the power grid.

In addition, as shown, the generator 28 is typically electricallycoupled to a bi-directional power converter 38 that includes arotor-side converter 40 joined to a line-side converter 42 via aregulated DC link 44. The rotor-side converter 40 converts the AC powerprovided from the rotor 30 into DC power and provides the DC power tothe DC link 44. The line side converter 42 converts the DC power on theDC link 44 into AC output power suitable for the power grid. Thus, theAC power from the power converter 38 can be combined with the power fromthe stator 32 to provide multi-phase power (e.g. three-phase power)having a frequency maintained substantially at the frequency of thepower grid (e.g. 50 Hz/60 Hz).

The illustrated three-winding transformer 34 typically has (1) a 33kilovolt (kV) medium voltage (MV) primary winding 33 connected to thepower grid, (2) a 6 to 13.8 kV MV secondary winding 35 connected to thegenerator stator 32, and (3) a 690 to 900 volt (V) low-voltage (LV)tertiary winding 37 connected to the line-side power converter 42.

Referring now to FIG. 3, individual power systems of a plurality of windturbines 10 may be arranged in a predetermined geological location andelectrically connected together to form a wind farm 46. Morespecifically, as shown, the wind turbines 10 may be arranged into aplurality of groups 48 with each group separately connected to a mainline 50 via switches 51, 52, 53, respectively. In addition, as shown,the main line 50 may be electrically coupled to another, largertransformer 54 for further stepping up the voltage amplitude of theelectrical power from the groups 48 of wind turbines 10 before sendingthe power to the grid.

With the growing success of wind power production in recent years, thisform of power has gained significant market share. As wind power is nota power source having a timely constant power output, but includesvariations, for example due to variations of the wind speed, operatorsof power distribution networks have to take this into account. One ofthe consequences is, for example, that the distribution and transmissionnetworks have become more difficult to manage. This pertains also to themanagement of the amount of reactive power flow in a network.

Referring now to FIGS. 4 and 5, schematic diagrams of a farm-levelreactive power control scheme and a turbine-level reactive power controlscheme are illustrated, respectively, according to conventionalconstruction. More specifically, as shown in FIG. 4, the farm-levelcontrol scheme includes a fast inner voltage magnitude loop 58 and aslow outer reactive power loop 56. Further, as shown, the farm-levelcontroller alternates between voltage control and reactive powercontroller via switch 64. For voltage control, the farm-level controllerreceives a voltage set point 66 and limits the set point via a slew ratelimiter 68. For reactive power control, the farm-level controllerregulates the reactive power via a VAR regulator 70 based on a reactivepower set point 72 and a reactive power feedback signal Q_(FBK). Thefarm-level controller then limits either the voltage or reactive powersignal that enters the fast inner voltage magnitude loop 58. As shown at74, another voltage regulator 74 regulates the voltage signal todetermine a reactive power command for the wind farm. As shown at 76,the farm-level controller then distributes the net reactive powercommand (Q_(CMD)) to individual wind turbines 102 (i.e. 10 ₁, 10 ₂, to10 _(n) and so on).

At the turbine level, as shown in FIG. 5, there is another volt/VARcontrol loop that consists of a faster inner magnitude loop 62 and aslower outer reactive power loop 60. Further, the three-windingtransformer 34 of each wind turbine 10 provides a certain impedance thatallows the wind turbines 10 in the wind farm 46 to regulate the voltageat the secondary winding of the three-winding transformer. This in turnenables regulating the voltage at the point of interconnection (POI) orthe point of common coupling (POCC). Thus, the faster inner magnitudeloop 62 provides the grid with fast voltage magnitude support fortransient events, while the slower outer reactive power loop 60 providesVAR balance between the wind turbines 10 in steady state.

In such systems, however, the three-winding transformers 34 associatedwith each wind turbine 10 is expensive. Particularly, the secondarywinding 35 of the transformer 34 that is connected to the generatorstator 32 can be costly. Thus, it would be advantageous to eliminatesuch three-winding transformers from wind turbine power systems. Theoutput of two or more wind turbines are directly coupled to mediumvoltage collection system together. The collection system then connectsthe wind turbines to the secondary winding of a cluster transformer thatsteps up the voltage from MV level to the POI voltage level. In thisconfiguration, the wind turbines are connected to a common point withoutany impedance between them. Due to the absence of impedance provided bythe stator winding 35 in the three-winding transformers 34, however, thegoal of each wind turbine to simply regulate the turbine terminalvoltage becomes difficult.

Thus, it would be advantageous to provide a wind farm having a pluralityof wind turbines without the three-winding transformer described above,but that maintains the systems' ability to control reactive power.

BRIEF DESCRIPTION

Aspects and advantages of the invention will be set forth in part in thefollowing description, or may be obvious from the description, or may belearned through practice of the invention.

In one aspect, the present subject matter is directed to a method foroptimizing reactive power generation of an electrical power system. Theelectrical power system has a plurality of clusters of electrical powersubsystems. Each cluster of electrical power subsystems is connected toa power grid via a separate cluster transformer. Each of the electricalpower subsystems has a power converter electrically coupled to agenerator. The method includes generating, via a plurality ofcluster-level controllers, a cluster-level reactive power command foreach cluster of electrical power subsystems based on a system-levelreactive power command. The method also includes determining, via thecluster-level controllers, a subsystem-level reactive power command foreach of the electrical power subsystems based on the cluster-levelreactive power command. Further, the method includes evaluating, via aplurality of subsystem-level controllers, reactive power capability of aplurality of reactive power sources within each of the electrical powersubsystems. Moreover, the method includes generating, via each of thesubsystem-level controllers, an actual reactive power for each of theelectrical power subsystems based on the evaluation by allocating aportion of the subsystem-level reactive power command to each of theplurality of reactive power sources.

In another aspect, the present disclosure is directed to an electricalpower system connected to a power grid. The electrical power systemincludes a system-level controller, a plurality of clusters ofelectrical power subsystems, a cluster transformer connecting eachcluster of electrical power subsystems to the power grid, a plurality ofcluster-level controllers communicatively coupled with the system-levelcontroller, and a plurality of subsystem-level controllerscommunicatively coupled with each of the cluster-level controllers. Eachof the electrical power subsystems includes a power converterelectrically coupled to a generator having a generator rotor and agenerator stator. Each of the electrical power subsystems defines astator power path and a converter power path for providing power to thepower grid. The converter power path includes a partial powertransformer. Each of the clusters of electrical power subsystems iscommunicatively coupled with one of the cluster-level controllers.Further, each of the cluster-level controllers is configured to performone or more operations, including but not limited to generating acluster-level reactive power command for each cluster of electricalpower subsystems based on a system-level reactive power command anddetermining a subsystem-level reactive power command for each of theelectrical power subsystems based on the cluster-level reactive powercommand. Moreover, each of the subsystem-level controllers is configuredto perform one or more operations, including but not limited toevaluating reactive power capability of a plurality of reactive powersources within each of the electrical power subsystems, and generatingan actual reactive power for each of the electrical power subsystemsbased on the evaluation by allocating a portion of the subsystem-levelreactive power command to each of the plurality of reactive powersources. It should be understood that the electrical power system mayfurther include any of the additional features as described herein.

In yet another aspect, the present disclosure is directed to a windfarm. The wind farm includes a plurality of wind turbine clusters eachhaving a plurality of wind turbines, a cluster transformer connectingeach cluster of wind turbines to a power grid, a cluster-levelcontroller communicatively coupled to each of the wind turbine clusters,and a plurality of turbine controllers communicatively coupled with eachof the cluster-level controllers. Each of the wind turbines has a powerconverter electrically coupled to a generator with a generator rotor anda generator stator. Further, each of the wind turbines defines a statorpower path and a converter power path for providing power to the powergrid. The converter power path contains a partial power transformer. Thecluster-level controllers are configured to perform one or moreoperations, including but not limited to generating a cluster-levelreactive power command for each cluster of electrical power subsystemsbased on a system-level reactive power command and determining asubsystem-level reactive power command for each of the electrical powersubsystems based on the cluster-level reactive power command. Further,each of the turbine controllers is configured to perform one or moreoperations, including but not limited to evaluating reactive powercapability of a plurality of reactive power sources within each of theelectrical power subsystems and generating an actual reactive power foreach of the electrical power subsystems based on the evaluation byallocating a portion of the subsystem-level reactive power command toeach of the plurality of reactive power sources. It should be understoodthat the wind farm may further include any of the additional features asdescribed herein.

These and other features, aspects and advantages of the presentinvention will become better understood with reference to the followingdescription and appended claims. The accompanying drawings, which areincorporated in and constitute a part of this specification, illustrateembodiments of the invention and, together with the description, serveto explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof, directed to one of ordinary skill in the art, is setforth in the specification, which makes reference to the appendedfigures, in which:

FIG. 1 illustrates a perspective view of a portion of one embodiment ofa wind turbine according to conventional construction;

FIG. 2 illustrates a schematic diagram of a conventional electricalpower system suitable for use with the wind turbine shown in FIG. 1;

FIG. 3 illustrates a schematic diagram of one embodiment of aconventional wind farm according to conventional construction,particularly illustrating a plurality of wind turbine power systems suchas those illustrated in FIG. 2 connected to a single sub-stationtransformer;

FIG. 4 illustrates a schematic diagram of one embodiment of a farm-levelcontrol scheme according to conventional construction;

FIG. 5 illustrates a schematic diagram of one embodiment of aturbine-level control scheme according to conventional construction;

FIG. 6 illustrates a schematic diagram of one embodiment of anelectrical power system for a wind turbine according to the presentdisclosure;

FIG. 7 illustrates a schematic diagram of one embodiment of a wind farmaccording to the present disclosure, particularly illustrating aplurality of wind turbine clusters each connected to the grid via acluster transformer;

FIG. 8 illustrates a block diagram of one embodiment of a wind turbinecontroller according to the present disclosure;

FIG. 9 illustrates a schematic diagram of one embodiment of a farm-levelcontrol scheme according to the present disclosure;

FIG. 10 illustrates a schematic diagram of one embodiment of acluster-level control scheme according to the present disclosure;

FIG. 11 illustrates a flow diagram of one embodiment of a method foroptimizing reactive power generation of an electrical power systemaccording to the present disclosure; and

FIG. 12 illustrates a schematic diagram of one embodiment of a turbinecontroller having a selector for selecting between reactive powersources based on availability for optimizing reactive power generationof a wind farm according to the present disclosure.

DETAILED DESCRIPTION

Reference now will be made in detail to embodiments of the invention,one or more examples of which are illustrated in the drawings. Eachexample is provided by way of explanation of the invention, notlimitation of the invention. In fact, it will be apparent to thoseskilled in the art that various modifications and variations can be madein the present invention without departing from the scope or spirit ofthe invention. For instance, features illustrated or described as partof one embodiment can be used with another embodiment to yield a stillfurther embodiment. Thus, it is intended that the present inventioncovers such modifications and variations as come within the scope of theappended claims and their equivalents.

Generally, the present subject matter is directed to a method forcontrolling a wind farm having a plurality of clusters of wind turbineswith a cluster transformer connecting each cluster of wind turbines to apower grid. The method includes receiving, via a plurality ofcluster-level controllers, a reactive power command from a farm-levelcontroller. The method also includes generating, via the cluster-levelcontrollers, a cluster-level reactive current command for each clusterof wind turbines based on the reactive power command. Further, themethod includes distributing, via the cluster-level controllers, aturbine-level reactive current command to turbine-level controllers ofthe wind turbines based on the cluster-level reactive current command.

As such, the system and method of the present disclosure provide manyadvantages not present in the prior art. For example, in the proposedtopology, the voltage/VAR control loop at the turbine-level iseliminated. Instead, the same is implemented at the cluster level,thereby eliminating issues associated with all wind turbines in thecluster regulating the same terminal voltage. As such, the cluster-levelcontrollers receive the reactive power command from the plant controllerand generate the commands for the reactive component of the turbinecurrents. The system and method of the present disclosure also preventsvolt oscillation or transient voltage stability, allowing the system tooperate properly and improving power system reliability, therebyenabling elimination of the three-winding main transformer, which inturn, helps in enabling lower wind turbine and balance of plant costs,higher efficiency, higher annual energy production, and/or space savingdesigns.

Referring now to FIG. 6, a schematic diagram of one embodiment of anelectrical power subsystem 102 according to the present disclosure isillustrated. It should be understood that the term “subsystem” is usedherein to distinguish between the individual power systems (e.g. asshown in FIG. 6) and the overall electrical power system 105 of FIG. 7that includes a plurality of electrical power subsystems 102. Those ofordinary skill in the art, however, will recognize that the electricalpower subsystem 102 of FIG. 6 may also be referred to more generically,such as a simply a system (rather than a subsystem). Therefore, suchterms may be used interchangeably and are not meant to be limiting.

Further, as shown, the electrical power subsystem 102 may correspond toa wind turbine power system 100. More specifically, as shown, the windturbine power system 100 includes a rotor 104 that includes a pluralityof rotor blades 106 attached to a rotating hub 108. As wind impacts therotor blades 106, the blades 106 transform wind energy into a mechanicalrotational torque that rotatably drives a low-speed shaft 110. Thelow-speed shaft 110 is configured to drive a gearbox 112 thatsubsequently steps up the low rotational speed of the low-speed shaft110 to drive a high-speed shaft 114 at an increased rotational speed.The high-speed shaft 114 is generally rotatably coupled to a doubly-fedinduction generator 116 (referred to hereinafter as DFIG 116) so as torotatably drive a generator rotor 118. As such, a rotating magneticfield may be induced by the generator rotor 118 and a voltage may beinduced within a generator stator 120 that is magnetically coupled tothe generator rotor 118. In one embodiment, for example, the generator116 is configured to convert the rotational mechanical energy to asinusoidal, three-phase alternating current (AC) electrical energysignal in the generator stator 120. Thus, as shown, the associatedelectrical power can be transmitted from the generator stator 120directly the grid.

In addition, as shown, the generator 116 is electrically coupled to abi-directional power converter 122 that includes a rotor-side converter124 joined to a line-side converter 126 via a regulated DC link 128.Thus, the rotor-side converter 124 converts the AC power provided fromthe generator rotor 118 into DC power and provides the DC power to theDC link 128. The line side converter 126 converts the DC power on the DClink 128 into AC output power suitable for the power grid. Morespecifically, as shown, the AC power from the power converter 122 can becombined with the power from the generator stator 120 via a converterpower path 127 and a stator power path 125, respectively. For example,as shown, and in contrast to conventional systems such as thoseillustrated in FIGS. 1-3, the converter power path 127 may include apartial power transformer 130 for stepping up the voltage amplitude ofthe electrical power from the power converter 122 such that thetransformed electrical power may be further transmitted to the powergrid. Thus, as shown, the illustrated system 102 of FIG. 6 does notinclude the conventional three-winding main transformer described above.Rather, as shown in the illustrated embodiment, the partial powertransformer 130 may correspond to a two-winding transformer having aprimary winding 132 connected to the power grid and a secondary winding134 connected to the rotor side converter 124.

In addition, the electrical power system 100 may include one or morecontrollers. For example, the system 100 may include a system-levelcontroller (e.g. a farm-level controller 107), one or more cluster-levelcontrollers 176, and/or one or more subsystem-level controllers (e.g.turbine-level controllers 136). As such, the various controllersdescribed herein are configured to control any of the components of thewind farm 105, the wind turbine clusters 137, and/or the individual windturbines 100 and/or implement the method steps as described herein. Forexample, as shown particularly in FIG. 8, a block diagram of oneembodiment of a controller as described herein is illustrated. As shown,the controller may include one or more processor(s) 138 and associatedmemory device(s) 140 configured to perform a variety ofcomputer-implemented functions (e.g., performing the methods, steps,calculations and the like and storing relevant data as disclosedherein). Additionally, the controller may also include a communicationsmodule 142 to facilitate communications between the controller and thevarious components of the wind farm 105, e.g. any of the components ofFIGS. 6 and 7. Further, the communications module 142 may include asensor interface 144 (e.g., one or more analog-to-digital converters) topermit signals transmitted from one or more sensors 139, 141, 143 to beconverted into signals that can be understood and processed by theprocessors 138. It should be appreciated that the sensors 139, 141, 143may be communicatively coupled to the communications module 142 usingany suitable means. For example, as shown in FIG. 8, the sensors 139,141, 143 may be coupled to the sensor interface 144 via a wiredconnection. However, in other embodiments, the sensors 139, 141, 143 maybe coupled to the sensor interface 144 via a wireless connection, suchas by using any suitable wireless communications protocol known in theart. As such, the processor 138 may be configured to receive one or moresignals from the sensors 139, 141, 143.

As used herein, the term “processor” refers not only to integratedcircuits referred to in the art as being included in a computer, butalso refers to a controller, a microcontroller, a microcomputer, aprogrammable logic controller (PLC), an application specific integratedcircuit, and other programmable circuits. The processor 138 is alsoconfigured to compute advanced control algorithms and communicate to avariety of Ethernet or serial-based protocols (Modbus, OPC, CAN, etc.).Additionally, the memory device(s) 140 may generally comprise memoryelement(s) including, but not limited to, computer readable medium(e.g., random access memory (RAM)), computer readable non-volatilemedium (e.g., a flash memory), a floppy disk, a compact disc-read onlymemory (CD-ROM), a magneto-optical disk (MOD), a digital versatile disc(DVD) and/or other suitable memory elements. Such memory device(s) 140may generally be configured to store suitable computer-readableinstructions that, when implemented by the processor(s) 138, configurethe controller to perform the various functions as described herein.

In operation, alternating current (AC) power generated at the generatorstator 120 by rotation of the rotor 104 is provided via a dual path tothe grid, i.e. via the stator power path 125 and the converter powerpath 127. More specifically, the rotor side converter 124 converts theAC power provided from the generator rotor 118 into DC power andprovides the DC power to the DC link 128. Switching elements (e.g.IGBTs) used in bridge circuits of the rotor side converter 124 can bemodulated to convert the AC power provided from the generator rotor 118into DC power suitable for the DC link 124. The line side converter 126converts the DC power on the DC link 128 into AC output power suitablefor the grid. In particular, switching elements (e.g. IGBTs) used inbridge circuits of the line side converter 126 can be modulated toconvert the DC power on the DC link 128 into AC power. As such, the ACpower from the power converter 122 can be combined with the power fromthe generator stator 120 to provide multi-phase power having a frequencymaintained substantially at the frequency of the bus. It should beunderstood that the rotor side converter 124 and the line side converter126 may have any configuration using any switching devices thatfacilitate operation of electrical power system 200 as described herein.

Further, the power converter 122 may be coupled in electronic datacommunication with the turbine controller 136 and/or a separate orintegral converter controller 154 to control the operation of the rotorside converter 124 and the line side converter 126. For example, duringoperation, the controller 136 may be configured to receive one or morevoltage and/or electric current measurement signals from the first setof voltage and electric current sensors 139, 141, 143. Thus, thecontroller 136 may be configured to monitor and control at least some ofthe operational variables associated with the wind turbine 100 via thesensors 139, 141, 143. In the illustrated embodiment, the sensors 139,141, 143 may be electrically coupled to any portion of electrical powersubsystem 102 that facilitates operation of electrical power subsystem102 as described herein.

It should also be understood that any number or type of voltage and/orelectric current sensors may be employed within the wind turbine 100 andat any location. For example, the sensors may be current transformers,shunt sensors, rogowski coils, Hall Effect current sensors, MicroInertial Measurement Units (MIMUs), or similar, and/or any othersuitable voltage or electric current sensors now known or laterdeveloped in the art. Thus, the converter controller 154 is configuredto receive one or more voltage and/or electric current feedback signalsfrom the sensors 139, 141, 143. More specifically, in certainembodiments, the current or voltage feedback signals may include atleast one of line feedback signals, line-side converter feedbacksignals, rotor-side converter feedback signals, or stator feedbacksignals.

Referring particularly to FIG. 7, individual power systems (such as thepower subsystem 102 illustrated in FIG. 4) may be arranged in at leasttwo clusters 137 to form an electrical power system 105. Morespecifically, as shown, the wind turbine power systems 100 may bearranged into a plurality of clusters 137 so as to form a wind farm.Thus, as shown, each cluster 137 may be connected to a separatetransformer 145, 146, 147 via switches 150, 151, 152, respectively, forstepping up the voltage amplitude of the electrical power from eachcluster 137 such that the transformed electrical power may be furthertransmitted to the power grid. In addition, as shown, the transformers145, 146, 147 are connected to a main line 148 that combines the voltagefrom each cluster 137 before sending the power to the grid. Further, asmentioned, each of the clusters 137 may be communicatively coupled witha cluster-level controller 176, e.g. as shown in FIG. 11 and furtherdiscussed below, that controls each of the transformers 145, 145, 147.In addition, as shown, the wind farm 105 may include one or moreautomatic voltage regulators (e.g. tap changers 171) arranged with eachof the transformers 145, 145, 147 and/or one or more reactor powerdevices 173. For example, the reactor power devices 173 may include anyone of the following: a capacitor bank 175, a reactor bank 177, and/or astatic synchronous compensator (STATCOM) 179.

Referring now to FIGS. 9-10, various illustrations are provided tofurther describe the systems and methods of the present disclosure. Forexample, FIG. 9 illustrates a schematic diagram of one embodiment of afarm-level control scheme according to the present disclosure; and FIG.10 illustrates a schematic diagram of one embodiment of a cluster-levelcontrol scheme according to the present disclosure that address theissues associated with the turbine-level volt-VAR loop illustrated inFIG. 9. More specifically, as shown in FIG. 10, reactive power controlis implemented at the cluster level. As such, the control scheme of thepresent disclosure eliminates issues associated with wind turbines 100in the cluster 137 regulating the same terminal voltage. Rather, thecluster-level controllers 176 receive the reactive power command fromthe farm-level controller 107 and generate the commands for the reactivecomponent of the turbine currents.

More specifically, as shown, the farm-level control scheme includes afast inner voltage magnitude loop 155 and a slow outer reactive powerloop 153. Further, as shown, the farm-level controller 107 alternatesbetween voltage control and reactive power controller via switch 164.For voltage control, the farm-level controller 107 receives a voltageset point 156 and limits the set point via a slew rate limiter 158. Forreactive power control, the farm-level controller 107 regulates thereactive power via a reactive power (VAR) regulator 162 based on areactive power set point 160 and a reactive power feedback signalQ_(FBK), e.g. from the power grid, e.g. at the primary side (i.e. thehigh voltage) of the farm substation transformers 145, 146, or 147and/or at the secondary (i.e. medium voltage) of the farm substationtransformers 145, 146, or 147. The farm-level controller 107 then limitseither the voltage or reactive power signal that enters the fast innervoltage magnitude loop 155 via limiter 166. A voltage regulator 170 thenregulates the voltage signal 169 to determine a reactive power command(i.e. Q_(FARMCMD) 172) for the wind farm 100. Thus, as shown at 174, thefarm-level controller 107 then distributes the net reactive powercommand (Q_(CMD)) to each of the cluster-level controller 176.

Referring now to FIG. 10, at the cluster-level, the cluster controllers176 are configured to receive a reactive power command (i.e. Q_(CMD))from the farm-level controller 107. In addition, as shown, thecluster-level controllers 176 may also receive a reactive power feedbacksignal (e.g. Q_(FBK)) and determine a reactive power error 178 as afunction of the reactive power command Q_(CMD) for each cluster 137 andthe reactive power feedback signal Q_(FBK). Further, the cluster-levelcontrollers 176 also generate a cluster-level reactive power command 190for each cluster 137 of electrical power subsystems 102 based on thereactive power error 178. More specifically, in certain embodiments, thecluster-level controllers 176 may include a reactive power (VAR)regulator 180 configured generate a first output 182 based on thereactive power error 178. For example, in certain embodiments, thereactive power regulator 180 may include a proportional integral (PI)controller, a proportional derivative (PD) controller, a proportionalintegral derivative (PID) controller, a state space controller, oranother other suitable controller.

In further embodiments, as shown, the cluster-level controllers 176 mayeach include a limiter 183 configured to limit the first output 182 fromthe VAR regulator 180, e.g. based on a maximum voltage condition and aminimum voltage condition to obtain a voltage value 184. As such, thecluster-level controllers 176 may also receive a voltage feedback signalV_(FBK) from a secondary winding of the associated cluster transformer145, 146, 147 or point of common coupling (designated in the figures asPOI) and determine a voltage error 185 as a function of the voltagevalue 184 and the voltage feedback V_(FBK). In addition, eachcluster-level controller 176 may include a voltage regulator 186configured to generate a second output 187 based on the voltage error185. For example, in certain embodiments, the voltage regulator 186 mayinclude a proportional integral (PI) controller, a proportionalderivative (PD) controller, a proportional integral derivative (PID)controller, a state space controller, or another other suitablecontroller.

In addition, as shown, the cluster-level controllers 176 may eachinclude a limiter 188 configured to limit the second output 187 from thevoltage regulator 186, e.g. based on a maximum current condition and aminimum current condition to obtain the cluster-level reactive powercommand 190. Thus, as shown at 192, the cluster-level controllers 176distribute a subsystem-level reactive power command 194 to theturbine-level controllers 136 of the wind turbines 100 based on thecluster-level reactive current command 190.

Referring now to FIG. 11, a flow diagram of one embodiment of a method200 optimizing reactive power generation of a wind farm (e.g. such asthe wind farm 105 illustrated in FIG. 7) is illustrated according to thepresent disclosure. As shown at 202, the method 200 includes generating,via the cluster-level controllers 176, a cluster-level reactive powercommand 190 for each cluster 137 of wind turbines 100 based on afarm-level reactive power command Q_(CMD). As shown at 204, the method200 includes determining, via the cluster-level controllers 176, aturbine-level reactive power command 194 for each of the wind turbines100 based on the cluster-level reactive power command 190. As shown at206, the method 200 includes evaluating, via the plurality of turbinecontrollers 136, reactive power capability of a plurality of reactivepower sources within each of the wind turbines 100. For example, asshown in FIG. 12, the turbine controller(s) 136 is configured toevaluate the availability of a plurality of reactive power sources 196.Such reactive power sources 196, as illustrated, may include a turbinegenerator 197 (e.g. such as generator 116), a converter 198 (e.g. suchas power converter 122), or a modular VAR limit 199, as well as one ormore external reactive power sources installed within one or more of theplurality of clusters 137. For example, as mentioned, the externalreactive power sources may include the capacitor bank 175, the reactorbank 177, and/or the STATCOM 179.

In addition, the turbine controller(s) 136 may evaluate the reactivepower capability of the internal reactive power sources 196 within eachof the wind turbines 100 by determining available kilowatts (kW) andVARs for the reactive power sources 196 based on one or more turbineoperating parameters. In such embodiments, the turbine operatingparameter(s) may include, for example, temperature, active power,reactive power, voltage, current, phase angle, speed, or any otheroperating parameter. Thus, as shown in FIG. 12, the turbinecontroller(s) 136 may include also a selector 195 configured to selectthe reactive power source 196 capable of generating reactive power.

In such embodiments, the turbine controller(s) 136 may determine apriority ratio for each of the reactive power sources 196 within each ofthe wind turbines 100 based upon one or more weighting factors. As usedherein, the priority ratio indicates an amount of reactive power that acertain reactive power source can generate before another reactive powersource needs to start generating reactive power based on availability.The weighting factor(s) described herein may include, for example, acapability of the reactive power sources 196, a ratio of losses, or oneor more operating conditions of the reactive power sources 196. Thus,the selector 195 can select between the various reactive power sources196 based on their respective priority ratios.

Referring back to FIG. 11, as shown at 208, the method 200 includesgenerating, via each of the turbine controllers 136, an actual reactivepower for each of the wind turbines 100 based on the evaluation byallocating a portion of the turbine-level reactive power command to eachof the plurality of reactive power sources.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they include structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal languages of the claims.

What is claimed is:
 1. A method for optimizing reactive power generationof an electrical power system, the electrical power system having aplurality of clusters of electrical power subsystems, each cluster ofelectrical power subsystems connected to a power grid via a separatecluster transformer, each of the electrical power subsystems having apower converter electrically coupled to a generator, the methodcomprising: generating, via a plurality of cluster-level controllers, acluster-level reactive power command for each cluster of electricalpower subsystems based on a system-level reactive power command;determining, via the cluster-level controllers, a subsystem-levelreactive power command for each of the electrical power subsystems basedon the cluster-level reactive power command; evaluating, via a pluralityof subsystem-level controllers, reactive power capability of a pluralityof reactive power sources within each of the electrical powersubsystems; and, generating, via each of the subsystem-levelcontrollers, an actual reactive power for each of the electrical powersubsystems based on the evaluation by allocating a portion of thesubsystem-level reactive power command to each of the plurality ofreactive power sources.
 2. The method of claim 1, wherein evaluating,via the plurality of subsystem-level controllers, reactive powercapability of the plurality of reactive power sources within each of theelectrical power subsystems further comprises: determining availablekilowatts (kW) and VARs for the plurality of reactive power sourcesbased on one or more subsystem operating parameters.
 3. The method ofclaim 2, wherein the one or more subsystem operating parameters compriseat least one of temperature, active power, reactive power, voltage,current, phase angle, or speed.
 4. The method of claim 1, furthercomprising determining, via the subsystem-level controllers, a priorityratio for each of the plurality of reactive power sources within each ofthe plurality of electrical power subsystems based upon one or moreweighting factors, the priority ratio setting forth an amount ofreactive power that a certain reactive power source will generate beforeanother reactive power source will start to generate reactive power. 5.The method of claim 4, wherein the one or more weighting factorscomprises at least one of a capability of the reactive power sources, aratio of losses, or one or more operating conditions of the reactivepower sources.
 6. The method of claim 1, wherein the reactive powersources comprise at least one of the generator, the power converter, orone or more external reactive power sources installed within one or moreof the plurality of clusters.
 7. The method of claim 1, wherein thegenerator of each of the electrical power subsystems comprises adoubly-fed induction generator (DFIG).
 8. The method of claim 1, whereinthe electrical power system comprises a wind farm, and wherein theelectrical power subsystems comprise wind turbine power systems.
 9. Themethod of claim 1, wherein each of the electrical power subsystemsdefines a stator power path and a converter power path for providingpower to the power grid, the converter power path comprising a partialpower transformer.
 10. The method of claim 9, wherein the partial powertransformer comprises at least one of a two-winding transformer or athree-winding transformer.
 11. An electrical power system connected to apower grid, comprising: a system-level controller; a plurality ofclusters of electrical power subsystems, each of the electrical powersubsystems comprising a power converter electrically coupled to agenerator having a generator rotor and a generator stator, each of theelectrical power subsystems defining a stator power path and a converterpower path for providing power to the power grid, the converter powerpath comprising a partial power transformer; a cluster transformerconnecting each cluster of electrical power subsystems to the powergrid; a plurality of cluster-level controllers communicatively coupledwith the system-level controller, each of the clusters of electricalpower subsystems communicatively coupled with one of the cluster-levelcontrollers, each of the cluster-level controllers configured to performone or more operations, comprising: generating a cluster-level reactivepower command for each cluster of electrical power subsystems based on asystem-level reactive power command; and, determining a subsystem-levelreactive power command for each of the electrical power subsystems basedon the cluster-level reactive power command; and, a plurality ofsubsystem-level controllers communicatively coupled with each of thecluster-level controllers, each of the subsystem-level controllersconfigured to perform one or more operations, comprising: evaluatingreactive power capability of a plurality of reactive power sourceswithin each of the electrical power subsystems; and, generating anactual reactive power for each of the electrical power subsystems basedon the evaluation by allocating a portion of the subsystem-levelreactive power command to each of the plurality of reactive powersources.
 12. The electrical power system of claim 11, wherein evaluatingthe reactive power capability of the plurality of reactive power sourceswithin each of the electrical power subsystems further comprises:determining available kilowatts (kW) and VARs for the plurality ofreactive power sources based on one or more subsystem operatingparameters.
 13. The electrical power system of claim 12, wherein the oneor more subsystem operating parameters comprise at least one oftemperature, active power, reactive power, voltage, current, phaseangle, or speed.
 14. The electrical power system of claim 11, furthercomprising determining, via the subsystem-level controllers, a priorityratio for each of the plurality of reactive power sources within each ofthe plurality of electrical power subsystems based upon one or moreweighting factors, the priority ratio setting forth an amount ofreactive power that a certain reactive power source will generate beforeanother reactive power source will start to generate reactive power. 15.The electrical power system of claim 14, wherein the one or moreweighting factors comprises at least one of a capability of the reactivepower sources, a ratio of losses, or one or more operating conditions ofthe reactive power sources.
 16. The electrical power system of claim 11,wherein the reactive power sources comprise at least one of thegenerator, the power converter, or one or more external reactive powersources installed within one or more of the plurality of clusters. 17.The electrical power system of claim 11, wherein the generator of eachof the electrical power subsystems comprises a doubly-fed inductiongenerator (DFIG).
 18. The electrical power system of claim 11, whereinthe electrical power system comprises a wind farm, and wherein theelectrical power subsystems comprise wind turbine power systems.
 19. Theelectrical power system of claim 11, wherein the partial powertransformer comprises at least one of a two-winding transformer or athree-winding transformer.
 20. A wind farm, comprising: a plurality ofwind turbine clusters each comprising a plurality of wind turbines, eachof the wind turbines having a power converter electrically coupled to agenerator with a generator rotor and a generator stator, each of thewind turbines defining a stator power path and a converter power pathfor providing power to the power grid, the converter power pathcontaining a partial power transformer; a cluster transformer connectingeach cluster of wind turbines to a power grid; a cluster-levelcontroller communicatively coupled to each of the wind turbine clusters,the cluster-level controllers configured to perform one or moreoperations, comprising: generating a cluster-level reactive powercommand for each cluster of electrical power subsystems based on asystem-level reactive power command; and, determining a subsystem-levelreactive power command for each of the electrical power subsystems basedon the cluster-level reactive power command; and, a plurality of turbinecontrollers communicatively coupled with each of the cluster-levelcontrollers, each of the turbine controllers configured to perform oneor more operations, comprising: evaluating reactive power capability ofa plurality of reactive power sources within each of the electricalpower subsystems; and, generating an actual reactive power for each ofthe electrical power subsystems based on the evaluation by allocating aportion of the subsystem-level reactive power command to each of theplurality of reactive power sources.